Respuesta :

y=missing side:
p=16x+12
p=2(3x+8)+2y

16x+12=2(3x+8)+2y

Divide everything by 2:
8x+6=3x+8+y

-3x from each side:
5x+6=8+y

-8 from each side:
5x-2=y

y=5x-2

Hope this helps :)